1. Field of the Invention
The present invention relates generally to digital cellular mobile radio communication systems. The present invention relates more specifically to a method and system for effecting an improved mobile assisted handoff operation as the mobile station moves from one cell to another within a cellular communication network.
2. Description of the Related Art
In cellular mobile communication systems where a mobile station (MS) may be constantly moving, it is essential that the radio communications link with the mobile station be maintained when the station moves from one cell served by one base station (BS) to a second cell served by a different base station. The process of handing over the communications link from one base station to another, in a manner that is transparent to the user, is known as handoff.
The most commonly used criteria to determine the need for a handoff is received signal strength (RSS). In first generation cellular systems such as the Advanced Mobile Phone System (AMPS), the serving base station performs a received signal strength measurement on the signal received from the mobile station and uses this information to make a handoff decision. In such a system and method, there is no mobile station involvement in the measurement or decision making process.
Second generation digital cellular communication systems such as the IS-136 and GSM systems have introduced mobile assisted handoff (MAHO) where the mobile station assists the base station in the handoff decision making process by itself performing received signal strength and bit error rate (BER) measurements.
In the IS-136 system, the mobile station performs received signal strength and bit error rate measurements on the channel it is currently operating on and only received signal strength measurements on a list of candidate channels supplied to it by the base station. The RSS measurements on candidate channels are performed during the idle period between designated time slots in the communication link.
A technique called Digital Locate has also been developed by network infrastructure manufacturers that allows candidate base stations to perform received signal strength measurements on the mobile station in question and allows the base station to synchronize to the mobile station""s transmissions to verify the identity of the mobile station.
In the GSM system, the mobile station also performs received signal strength measurements during the idle period between designated time slots. In addition, since the frame structure of GSM has one idle time frame per multi-frame, during this idle time frame the mobile station synchronizes to the broadcast control channel (BCCH) of a candidate base station and reads the base station identification code (BSIC) to verify the identity of the base station. A number of patents, including U.S. Pat. Nos. 5,042,082, 5,200,957, and 5,257,401, describe systems within which the mobile station performs received signal strength measurements on a list of target base station channels. The base station then supplies the color code information of the mobile station to the target base stations and the target base stations measure the received signal strength of the mobile station in question and synchronize to the mobile station""s transmissions to read its color code.
Other patents, including U.S. Pat. No. 5,493,563, describe systems where measurements on voice channels of adjacent cells are performed. In addition, U.S. Pat. No. 5,157,661 describes a system where a communication test is performed on candidate channels during the free time of a time division multiplexed channel.
Despite the above efforts at improving the mobile assisted handoff operation, there are still serious drawbacks to the accuracy of the systems and their ability to identify and designate the most appropriate base station to receive the handoff. By involving the mobile station in the appropriate selection of a base station handoff, the accuracy improves, but remains a problem even with the kinds of measurements mentioned in the above patents.
It is therefore an object of the present invention to provide a system and method to improve the quality of the handoff decision by allowing the mobile station to synchronize to candidate base station transmissions for identity verification purposes, thus reducing the probability of a dropped call due to an erroneous handoff decision.
It is a further object of the present invention to provide a system wherein a mobile station has the ability to synchronize to candidate base station transmissions and to read the transmitted digital voice color code (DVCC).
It is a further object of the present invention to provide a method wherein a mobile station synchronizes to a candidate base station""s transmissions and reads the DVCC, thus assisting in verifying the identity of the base station.
It is a further object of the present invention to provide a system and method that avoid problems associated with strong co-channel interferers in the RSS measurements reported by a mobile station.
It is a further object of the present invention to incorporate into the mobile station certain functions currently performed by Digital Locate systems, thus permitting the elimination of separate Digital Locate radio transceivers.
It is a further object of the present invention to implement a system and method for improved mobile assisted handoff that requires a minimal amount of additional computational and memory resources in the mobile station.
In fulfillment of these and other objectives, the present invention provides a system and method that give the mobile station the ability to synchronize to candidate base station""s transmissions in order to read the transmitted digital voice color code (DVCC). This process is performed during the idle period between two designated time slots in a frame. The mobile station performing the process does not need advance synchronization information regarding the candidate handoff channels in order to read the DVCC. The invention is an enhancement to the existing mobile assisted handoff procedures described in the IS-136 standard. In the existing IS-136 standard, the mobile station performs a received signal strength measurement on candidate base station channels. The present invention improves on the use of received signal strength measurements in that it allows the mobile station to acquire and report information regarding the digital verification color code (DVCC) of the candidate base station channels. Since the DVCC uniquely identifies the cell site to which a channel belongs, it is used to distinguish the candidate base station channel (and its RSS measurements) from its co-channels (and their RSS measurements), allowing the network to make a more informed handoff decision.
In summary, the function of the improved mobile assisted handoff method is as follows:
Step 1: When mobile assisted handoff procedures are activated, the current base station issues a first measurement order containing a list of up to 24 channels for the mobile station to take signal strength measurements on.
Step 2: Upon receipt of the first measurement order, the mobile station begins to measure received signal strength and bit error rate on its current channel and received signal strength on the candidate channels listed in the first measurement order. These measurements are made during the mobile station""s idle time slots.
Step 3: The measurement results are reported back to the current base station in the channel quality message.
Step 4: Based on the measurement results reported in the channel quality message by the mobile station, the current base station selects a number of the most favorable channels, for example three (3), and issues a second measurement order to the mobile station with a list of the candidate handoff channels.
Step 5: Upon receipt of the second measurement order, the mobile station attempts to synchronize to each of the candidate channels during its idle time slots. The mobile station reads and decodes the digital verification color code to obtain the DVCC for each channel.
Step 6: The mobile station returns the measured values of each DVCC to the current base station.
Step 7: The network then makes a handoff decision based upon all of the information provided.